This invention relates to a bioactive compound and to compositions which contain it. The invention further relates to methods of microtubule stabilization. In particular, it relates to a compound which has cytotoxic properties and which therefore has utility in inter alia anti-tumor treatments.
In the search for anti-cancer drugs, compounds from natural sources, such as paclitaxel, extracted from the bark of the Pacific yew tree, have displayed useful anti-cancer activity and proven successful in clinical trials.
Marine sponges of the genus Mycale (Carmea) are a rich source of bioactive secondary metabolites of diverse structures. The mycalysines, mycalolides, deoxytedanolide and the macrolide pateamine have all been isolated from members of this genus and exhibit a variety of properties, including cytotoxic properties. See, for example, Perry et al., J. Am. Chem. Soc. (1988), 110, 4850-4851 and Northcote et al., Tetrahedron Letters (1991) 32, 6411-6414.
The strategy of using tubulin as a target for cancer chemotherapy is based on the increased growth and division of cancer cells and the fact that drugs that interfere with mitosis such as the vinca alkaloids that depolymerize microtubules have proven effective in the treatment of cancer. Paclitaxel (Taxol(copyright)) and taxotere (Docetaxel(copyright)) target tubulin but, unlike the vinca alkaloids and colchicine, cause polymerization and stabilization of microtubules. Both are currently used therapeutically for the treatment of solid tumors of the breast, ovary, and lung (He L., Orr G. A., Horwitz S. B., Drug Discovery Today (2001), 6, 1153-1164). Microtubule-stabilizing compounds can be divided into three groups:
(a) diterpenes, including the taxanes, paclitaxel and taxotere, isolated from Yew trees (He L. et al.) and eleutherobin/sarcodictyin, isolated from marine corals (Long B. H., Carboni J. M., Wasserman A. J., Cornell L. A., Casazza A. M., Jensen P. R., Lindel T., Fenical W., Fairchild C. R., Cancer Res. (1998), 58, 1111-1115);
(b) macrolides, including epothilones, isolated from the bacterium Sorangium cellulosum (Bollag D. M., McQueney P. A., Zhu J., Hensens O., Koupal L., Liesch J., Goetz M., Lazarides E., Woods C. M., Cancer Res. (1995), 55, 2325-2333 and Kowalski R. J., Giannakakou P., Hamel E., J. Biol. Chem. (1997), 272, 2534-2541) and laulimalides, isolated from the marine sponge Cacospongia mycofijiensis (Mooberry S. L., Tien G., Hernandez A. H., Plubrukarn A., Davidson B. S., Cancer Res. (1999), 59 653-660); and
(c) polyhydroxylated alkatetraene lactones, including discodermolide, isolated from a Caribbean sponge (Ter Haar E., Kowalski R. J., Hamel E., Lin C. M., Longley R. E., Gunasekera S. P., Rosenkranz H. S., Day B. W., Biochem. (1996), 35, 243-250 and Kowalski R. J., Giannakakou P., Gunasekera P., Longley R. E., Day B. W., Hamel E., Molec. Pharmacol. (1997), 52, 613-622).
The complex chemical syntheses required to produce clinically useful amounts of such drugs has limited their development as anti-cancer agents, although both epothilone and the more complex paclitaxel and taxotere have now been synthesized in sufficient amounts for clinical use. In addition, paclitaxel is lipophilic, thus having low aqueous solubility, and for clinical use, it must be dissolved in Cremaphor/ethanol, a vehicle that contributes to paclitaxel""s undesirable side effects that include hypersensitivity reactions, neutropenia, peripheral neuropathy, and alopecia (Bollag D. M. et al.). Paclitaxel""s hydrophobicity also promotes the acquisition of the multiple drug resistance (MDR) phenotype through expression of P-glycoprotein (P-gp) (Parekh H., Wiesen K., Simpkins H., Biochem. Pharmacol. (1997), 53, 461-470). P-gp is responsible for the efflux of a broad range of organic solutes from the cell, and paclitaxel is just one of these. In addition toover-expression of P-gp, some cells become resistant as a result of mutation of the paclitaxel binding site on xcex2-tubulin (Giannakakou P., Gussio R., Nogales E., Downing K. H., Zaharevitz D., Bollbuck B., Poy G., Sackett D., Nicolaou K. C., Fojo T., Proc. Nat. Acad. Sci. (USA) (2000), 97, 2904-2909).
Therefore there is a need for other microtubule-stabilizers with similar anti-mitotic activity to paclitaxel but which lack the interaction with P-gp or which bind to unique sites on the tubulin polymer. Epothilones, laulimalides, and discodermolides have shown promise in this area, displaying less loss of toxicity to certain P-gp-expressing cells than paclitaxel (Bollag D. M. et al., Kowalski R. J. et al., Mooberry S. L. et al. and Kowalski R. J. et al.), although still being transported to some extent by P-gp. At least three of the known microtubule-stabilizing drugs, the epothilones (Bollag D. M. et al. and Kowalski R. J. et al.), discodermolide (Kowalski R. J. et al.), and the eleutherobins (Long B. H.), compete with [3H]-paclitaxel for its binding site on xcex2-tubulin; however, epothilone and discodermolide also show different sensitivities to particular xcex2-tubulin mutations despite binding to a similar site. The paclitaxel binding site of xcex2-tubulin is available at 3.5 xc3x85 resolution (Nogales E. et al.), facilitating drug modeling approaches. A common pharmacophore has been partially described, but further structure/function studies are needed (He L. et al., Giannakakou P. et al., He L., Jagtap P. G., Kingston D. G. I., Shen H-J., Orr G. A., Horwitz S. B., Biochem. (2000), 39, 3972-3978 and Nicolaou K. C., Ritzxc3xa9n A., Namoto K., Chem. Comm. (2001), 17, 1523-1535).
Recent in vivo tests on tumor formation in nude mice have shown promise for desoxyepothilone analogues, specifically Z-12,13-desoxyepothilone B (dEpoB) and its more water soluble analogue, dEpoF (Chou T.-C., O""Conner O. A., Tong W. P., Guan Y., Zhang Z.-G., Stachel S. J., Lee C., Danishefsky S. J., Proc. Natl. Acad. Sci. (USA) (2001), 98, 8113-8118). Interestingly, the parent compound, epothilone B, although more potent than dEpoB or dEpoF, is too cytotoxic in vivo for use as an anti-cancer drug.
The applicants have now identified a bioactive compound from a marine sponge of the genus Mycale. The applicants have established the compound as a novel microtubule-stabilizing agent with potentially unique properties to the other known microtubule-stabilizing drugs. It is towards this compound, which the applicants have termed Peloruside A, to its functionally equivalent analogues, and to compositions, uses and methods of treatment which employ these compounds, that the present invention is broadly directed.
In a first aspect, the invention therefore provides a compound of formula (I); 
wherein R1, R2, R3, R4 and R5 are independently hydrogen, alkyl or acyl; or a functionally equivalent analogue thereof.
In a further aspect, the invention provides a compound of formula (II); 
wherein X is O or xe2x95x90C(R5)R6 and R1, R2, R3, R4, R5, and R6 are independently hydrogen, alkyl or acyl; or a functionally equivalent analogue thereof.
Preferably, the compound is of formula (III); 
or a functionally equivalent analogue thereof.
In another aspect, the invention provides a compound of formula (IV); 
or a functionally equivalent analogue thereof.
In another aspect, the invention provides a bioactive compound which has the NMR and/or IR spectral signature of FIGS. 1 and 2.
In another aspect, the invention provides composition which comprises a compound of the invention together with a suitable carrier therefor.
Preferably, the composition is a pharmaceutical composition.
In another further aspect, the invention provides a method of prophylaxis or therapy which comprises the step of administering to a patient in need of the same a compound or a composition of the invention.
Preferably, the prophylaxis or therapy is achieved by inhibiting the proliferation of cells.
Preferably, the compound is administered in an amount effective to provide microtubule stabilization.
A preferred method is a treatment of a patient against cancer.
The above formulae specify relative stereochemistry only.